Calling SSAValues `var` and "variables" is confusing

When I saw _"variables"_ in the docs,
As in:

> The IR can be viewed as a mapping from variables to statements, and indexing and iteration are consistent with that.

I assumed it was talking about **slots**.
But it isn't, it is talking about `SSAValue`.
(and `SSAValue`s never vary)

Start by locating the quoted documentation and usages of “variables” or “var” in the Julia project. Check how those terms relate to slots and SSAValue, then clarify the terminology consistently so readers do not confuse SSA values with mutable slots.

Written by the indexing model from the issue text.